The present invention relates to a variable displacement compressor for use in, for example, a vehicle air conditioner.
In general, variable displacement compressors are used in vehicle air conditioners. One of the compressors of this type is described in, for example, Japanese Second (examined) Pat. Publication No. 4-74549.
The disclosed compressor is called a wobble plate type variable displacement compressor. The compressor has a compressor casing defining a crank chamber therein. A rotor is disposed in the crank chamber and mounted on a main shaft. A swash plate is attached to the rotor via a hinge mechanism. The main shaft passes through the swash plate. Specifically, a sleeve is attached to the swash plate and receives the main shaft therethrough. A space is formed between an outer periphery of the sleeve and an inner periphery of the swash plate so that an inclination of the swash plate relative to the main shaft can be changed by means of the hinge mechanism.
A wobble plate is rotatably mounted on the swash plate via a bearing. A plurality of piston rods are coupled to the wobble plate through ball connection. The compressor casing is formed with a plurality of cylinders which are arranged at regular intervals so as to surround the main shaft. Each of the piston rods is coupled through ball connection to corresponding one of pistons disposed in the respective cylinders. In the crank chamber, a guide rod is supported by the compressor casing so as to extend in parallel to the main shaft. The guide rod is sandwiched by an end portion of the wobble plate so that the end portion of the wobble plate can wobble relative to the guide rod in an axial direction of the main shaft.
Following the rotation of the main shaft, the rotation of the rotor is transmitted to the swash plate so that the wobble plate wobbles to cause the pistons to make reciprocating motions. In this fashion, the compressing operation is carried out. As described above, since the inclination of the swash plate relative to the main shaft is changeable by means of the hinge mechanism, the piston stroke can be changed by controlling the inclination of the swash plate, thereby to change the compression displacement of the compressor.
In the foregoing wobble plate type variable displacement compressor, an open/close valve is provided in an air feed passage extending from a discharge chamber to the crank chamber for opening and closing the air feed passage so as to control a suction pressure at a given value. A bleed passage is further provided for constantly allowing discharge gas having flowed into the crank chamber through the air feed passage to escape into a suction chamber.
In the foregoing wobble plate type variable displacement compressor, when, for example, the suction pressure is higher than a set value of the open/close valve, the open/close valve is held closed so that the gas in the discharge chamber is not at all fed into the crank chamber while blowby gas introduced upon gas compression flows into the suction chamber via the bleed passage. Thus, a pressure differential between the crank chamber and the suction chamber is very small to cause the compressor to be operated at the maximum displacement.
In that event, lubrication of machine parts in the crank chamber relies on oil staying in the crank chamber. However, since the oil staying in the crank chamber is splashed to the inner periphery of the crank chamber due to centrifugal forces generated by the rotation of the main shaft and the swash plate, the lubrication of the bearings and the sliding portions near the main shaft becomes insufficient. Thus, depending on the compressor operating condition and the oil stay amount, abrasion or breakage of those machine parts may be caused.